Molten pool geometry, whose surface parameters may be extrapolated through direct process observations, has been identified as a fundamental indicator of stability in laser powder bed fusion (LPBF). However, a parameter that cannot be directly measured on industrial systems by means of conventional sensing equipment is the molten pool depth. Indeed, methods based on x-ray imaging demonstrated in the literature have helped to better understand the process. However, retrofitting such solutions to industrial systems does not appear as a viable route currently. Within the present investigation, a nonintrusive sensing method for the indirect measurement of subsurface molten pool geometry based on the detection of surface oscillations is presented. The analysis of frames acquired using a high-speed camera and a secondary illumination light allows the identification of the crests of capillary waves through bright reflections on the surface of the molten pool. The characteristic oscillation frequency of the surface ripples may be correlated with the penetration depth or to other subsurface geometrical parameters. Proof of concept testing of the sensing principle was conducted on two different materials, namely, AISI316L and IN718, by means of single track LPBF depositions. Experiments were conducted at different levels of laser emission power to induce variations in molten pool characteristics. The process was observed by employing an off-axis illumination light and a high-speed camera, which allowed acquisitions with high spatial and temporal resolution. The acquired frames were postprocessed to extract the oscillation indicator, and analysis of the power spectral density of the signal allowed for the identification of the oscillation frequency. Results show that oscillation frequencies range from 3 to 5.5 kHz. Molten pool penetration depth and cross-sectional area could be correlated with the oscillation frequencies for the inline detection of these parameters during LPBF depositions. For both materials, higher oscillation frequencies corresponded to a shallower molten pool and a smaller mass of molten material. Moreover, different characteristic curves of oscillation frequency variations as a function of the melt pool cross-sectional area were determined for IN718 and AISI316L.

中文翻译：

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通过感测激光粉末床熔合中的表面振动，非侵入性地估算熔池的地下几何属性

熔池的几何形状（其表面参数可以通过直接的过程观察来推断）已被确定为激光粉末床熔合（LPBF）稳定性的基本指标。但是，不能通过常规传感设备在工业系统上直接测量的参数是熔池深度。实际上，文献中证明的基于X射线成像的方法有助于更好地理解该过程。但是，将这样的解决方案改造为工业系统目前看来并不是可行的方法。在本研究中，提出了一种基于表面振动检测的间接测量地下熔池几何形状的非侵入式传感方法。通过使用高速摄像机和辅助照明灯对帧进行分析，可以通过熔池表面的明亮反射识别毛细管波的波峰。表面波纹的特征振荡频率可以与穿透深度或其他地下几何参数相关。借助单道LPBF沉积对AISI316L和IN718这两种不同的材料进行了传感原理的概念验证测试。在不同水平的激光发射功率下进行实验，以引起熔池特性的变化。通过使用离轴照明灯和高速照相机观察了该过程，从而可以进行具有高空间和时间分辨率的采集。对采集到的帧进行后处理，以提取振荡指示符，并对信号的功率谱密度进行分析以识别振荡频率。结果表明，振荡频率范围为3至5.5 kHz。熔池渗透深度和横截面积可与在LPBF沉积过程中在线检测这些参数的振荡频率相关。对于这两种材料，较高的振荡频率对应于较浅的熔池和较小质量的熔融材料。此外，对于IN718和AISI316L，确定了作为熔池横截面积的函数的振荡频率变化的不同特性曲线。对信号的功率谱密度进行分析并确定振荡频率。结果表明，振荡频率范围为3至5.5 kHz。熔池渗透深度和横截面积可与在LPBF沉积过程中在线检测这些参数的振荡频率相关。对于这两种材料，较高的振荡频率对应于较浅的熔池和较小质量的熔融材料。此外，对于IN718和AISI316L，确定了作为熔池横截面积的函数的振荡频率变化的不同特性曲线。对信号的功率谱密度进行分析，以识别振荡频率。结果表明，振荡频率范围为3至5.5 kHz。熔池渗透深度和横截面积可与在LPBF沉积过程中在线检测这些参数的振荡频率相关。对于这两种材料，较高的振荡频率对应于较浅的熔池和较小质量的熔融材料。此外，对于IN718和AISI316L，确定了作为熔池横截面积的函数的振荡频率变化的不同特性曲线。熔池渗透深度和横截面积可与在LPBF沉积过程中在线检测这些参数的振荡频率相关。对于这两种材料，较高的振荡频率对应于较浅的熔池和较小质量的熔融材料。此外，对于IN718和AISI316L，确定了作为熔池横截面积的函数的振荡频率变化的不同特性曲线。熔池渗透深度和横截面积可与在LPBF沉积过程中在线检测这些参数的振荡频率相关。对于这两种材料，较高的振荡频率对应于较浅的熔池和较小质量的熔融材料。此外，对于IN718和AISI316L，确定了作为熔池横截面积的函数的振荡频率变化的不同特性曲线。